The present invention relates to an optical transmitter and, more particularly, to an optical transmitter comprising an optical light source, such as a light emitting diode (LED) or a laser diode (LD), having a predetermined roll-off characteristic that cooperates with a bandpass amplifier having a slope characteristic which is selected to be complementary to the roll-off characteristic of the LED so as to yield a high frequency bandwidth transmitter.
Optical transmitters are finding use in advanced computer architectures, such as those found in avionic applications, that provide the means to transmit a larger amount of data over optical fibers located between processing elements. One of the major obstacles for such transmitters is the speed of the optical source of the transmitter itself. Laser diodes are commonly used to serve as a light source and are capable of high modulation rates, but suffer from well-known temperature problems and, therefore, normally require complex drive, control, and cooling circuitry.
Light emitting diodes (LEDs) also serve as light sources for the transmitter and do not suffer the same temperature restrictions as laser diodes. However, currently available LEDs are limited in their bandwidth capabilities to about several hundred megahertz (MHz). Efforts have been made to increase the bandwidth by increasing the speed of the LED itself. This increase in speed may be realized by decreasing the size of the active region of the LED or by more heavily doping the LED. These methods do improve the optical bandwidth of the LED but limit the amount of optical power that the LED can deliver.
Further efforts involving specialized drive circuits, based on a resistance-capacitive (RC) speed-up technique, have been expended to increase the power delivered by the LED light source. The RC speed-up technique is similar to the arrangements found in flip-flop circuits in which a capacitor is connected in parallel with a cross-coupling resistor both of which cooperate to accelerate the transition of the flip-flop from one stable state to another. While such drive circuits do increase the amount of the deliverable power, the obtainable bandwidth is limited to the hundreds of megahertz range.
High speed optical avionics data links have been constructed from multiple, relatively low speed, LEDs that are arranged in parallel. One such data link employs four LEDs, each operating at a speed of 250 MHz, and each arranged in parallel so that the overall bandwidth of the data link is in the gigahertz frequency range. Such a multiple arrangement involves electronics, in groups of four, at each end of the data link that include multiplexers, encoders, receivers, and clock recovery devices that accommodate the four LEDs. In addition, since the data on each line (related to the respective LED) may not arrive simultaneously (due to different path lengths), such circuit arrangements require buffer devices to store the incoming data for the subsequent use thereof.
Further efforts to increase the speed of optical transmitter are being pursued by the way of the so called "three terminal LEDs." These devices are being developed based on a quantum confined Stark effect and theoretically may achieve very high speeds. However, these LED devices are still in experimental stage and do not now represent a feasible solution for supplying a high speed optical source for an optical transmitter. It is desired that a conventional LED device be used as the optical source of the optical transmitter and provide for a bandwidth of operation in the gigahertz range. It is further desired that other optical emitters, such as a laser diode, serve as the optical source and provide for operation in the gigahertz range.